In association with wide developments of CMOS image sensors, image sensors that have electronic shutter function, which use a feature of CMOS integrated circuits, are being progressed (see Patent Literature (PTL) 1). Also, three-dimensional sensors, which contain range-measurement elements of MOS architecture or charge coupling element (CCD) architecture, are proposed (see PTLs 2 and 3). On the other hand, the CCD image sensors have an advantage of a high speed charge transfer and an advantage that charges can be transferred through multiple stages without any increase in noise, which do not exist in the CMOS image sensors. So, the development of the image sensor that has the merits of both of the CCD image sensor and the CMOS image sensor is expected. Moreover, in image sensors having the electronic shutter function, range-measurement elements, or three-dimensional sensors, a scheme is required, in which potentials just under gate electrodes are controlled in longitudinal direction (vertical direction) by using MOS structures so that signal charges can be transferred (see PTL 4).
On the other hand, in order to scan a reading-target at a high speed, a CCD image sensor having time-delay integration (TDI) function is proposed, in which a plurality of CCDs are arrayed on a line to form CCD-column, and the line configuration of CCD-columns are further deployed in a orthogonal direction to the direction of line (see PTL 5). In the TDI CCD, a picture obtained by first CCD-column is transferred as its original state to the second CCD-column. In the second CCD-column, a picture obtained by the second CCD-column is added to the picture sent from a former CCD-column, and the added picture is stored in the second CCD-column, and the stored picture is further transferred to the third CCD-column. In nth CCD-column, a picture obtained by the nth CCD-column is added to the picture transferred from (n−1)th CCD-column, and then transferred to (n+1)th CCD-column. In a case of a target that is moving at a constant speed along a constant direction, by matching the direction and speed of a movement of the imaging target with the direction and speed of charge transfers in the CCD-columns, respectively, the target can be exposed and imaged repeatedly for a number of times corresponding to the number of vertical stages along the orthogonal direction of the CCD-columns. That is, in the TDI sensor in which x stages CCD-columns are deployed, the obtained pictures are x times stored. Thus, the shutter speed can be made higher, and the image without any blurring can be obtained. Hence, it is possible to expect the realization of the image that is sufficient in brightness and high in sensibility.
However, in the case that the MOS structure is used to control the potential just beneath the gate electrode in the longitudinal direction (vertical direction), because interface defects and interface states are generated at an interface between a gate oxide film and a silicon surface, there was a problem of noises generated in a transfer path. Because of such noise problem ascribable to interface defects, interface states and the like, and problem of generation of dark currents, various schemes of buried channel structures and the like are tried historically to solve those problems in the CCD image sensors and the TDI CCDs in view of earlier historical development phase.
Even in a time of flight (TOF) range sensor in which a flight time of light is used to obtain a range image, the MOS structure is used to control the potential just beneath the gate electrode in the longitudinal direction (vertical direction). For example, a CMOS range-measurement element and a TOF image sensor which uses the CMOS range-measurement element are proposed, the structure of the CMOS range-measurement element encompasses an n-type charge-generation buried region, an n-type charge-transfer buried region and an n-type charge-read-out buried region, which are buried on p-type semiconductor layer, and an insulating film covering upper portions of the charge-generation buried region, the charge-transfer buried region and the charge-read-out buried region, a transfer-gate electrode arranged on the insulating film, formed to transfer the signal charges to the charge-transfer buried region, and a read-out gate electrode arranged on the insulating film, formed to transfer the signal charges to the charge-read-out buried region (see PTL 6). In the charge-generation buried region of the CMOS range-measurement element, when a light-pulse is received, in the semiconductor layer just under the charge-generation buried region, an optical signal is converted into the signal charges, and a distance from a target is consequently measured on the basis of a distribution ratio of the charges stored in the charge-transfer buried region. Even in the CMOS range-measurement element and the TOF image sensor that uses the CMOS range-measurement elements, there is the fears of the problem of the noises caused by interface defects, interface states and the like, just under the transfer-gate electrode, and the problem of the generation of the dark current. Moreover, in a case of using the transfer-gate electrode as described in PTL 6, it is difficult to control a potential inclination over a long distance. Thus, over the long distance along a charge-transfer direction, it was actually unreasonable to make an electric field substantially constant. For this reason, in the semiconductor elements such as the range-measurement elements, which had a long charge-transfer path, troubles were generated in which, since carriers were stopped in the course of the charge-transfer path, it was difficult to achieve the expected performances.
By the way, method for manufacturing the conventional CCD image sensors and method for manufacturing the CMOS image sensors greatly differ from each other. Although there is a report of an image sensor in which both of CCD and CMOS image sensors CCD and CMOS image sensors are mixed, a procedure for manufacturing the mixed structure of CCD and CMOS image sensors becomes long, and the number of photo masks for manufacturing the mixed structure also increases. Thus, there is a problem with regard to a manufacturing cost of the mixed structure.